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Software Architecture Guide

Scatter/Gather

Explanation

Section titled “Explanation”

Scatter-Gather is an integration pattern. It broadcasts a request to several recipients. It then merges their replies into one message.

Two parts. The “scatter” part sends the request out. The “gather” part uses an Aggregator. An Aggregator is a component that collects the replies and distills them into one response message.

Two ways to reach recipients.

  • A Publish/Subscribe Channel (see the article of that name) lets the pattern use a single channel. The cost is that it gives up control over who receives the request.
  • A Recipient List (see the article of that name) lets the pattern control which recipients are addressed.

When to stop waiting. The aggregator needs a rule for when it is done.

  • Wait for all: every reply must arrive. One missing or late reply blocks the whole result.
  • Timeout: wait a set time, then decide using the replies received.
  • First best: take the fastest reply and ignore the rest.

In practice. Spring Integration offers this as one combined endpoint. It pairs a publish/subscribe channel or a recipient list router with an aggregator. It waits a bounded time for the aggregated reply. If time runs out, it returns null. A finite timeout therefore avoids waiting forever.

Example

Section titled “Example”

A travel site asks three hotel booking systems for a price on the same dates. Each system answers on its own schedule. The site waits two seconds. It then combines whatever arrived into one answer for the visitor. A system that did not answer in time is left out.

Trade-offs

Section titled “Trade-offs”
  • Waiting for all replies is simple, but one slow or missing reply blocks everything.
  • A timeout keeps the caller moving, but the result may rest on fewer replies than hoped.
  • Taking only the fastest reply is quick, but it ignores all other replies.
  • A publish/subscribe channel needs only one channel, but you lose control over the recipients. A recipient list keeps that control.

A request is scattered to several recipients. An aggregator gathers the replies into one response.

Diagram source 
flowchart LR
  C[Client] -->|request| S[Scatter]
  S --> R1[Recipient A]
  S --> R2[Recipient B]
  S --> R3[Recipient C]
  R1 --> A[Aggregator]
  R2 --> A
  R3 --> A
  A -->|single response| C

Checks

Section titled “Checks”
  • The document states how long the aggregator waits for replies, such as a timeout, and what happens when it expires.
  • The document says whether a missing or delayed reply can block the combined result.
  • The document states whether recipients are chosen through a Publish/Subscribe Channel or through a Recipient List.
  • The document names the component that combines the replies into one response.
Section titled “Related”

Sources

Section titled “Sources”